Controlled collapse chip connection (C4) or flip-chip technology has been successfully used for over twenty years for interconnecting high I/O (input/output) count and area array solder bumps on silicon chips to base ceramic chip carriers, for example alumina carriers. In C4 technology or flip chip packaging, one or more integrated circuit chips are mounted above a single or multiple layer ceramic (MLC) substrate or board, and pads on the chip(s) are electrically or mechanically connected to corresponding pads on the other substrate by a plurality of electrical connections, such as solder bumps.
In MLC packages, a ceramic substrate is the platform upon which chips, passive components, protective lids, and thermal enhancement hardware are attached according to well known techniques. Wiring patterns within the substrate carrier define escape paths in single chip modules (SCMs) and multichip modules (MCMs), transforming the tight I/O pitch at the die level of the chips to a workable pitch at the board level. The wiring pattern also establishes the modules' power distribution network. Vertical metal vias provide interconnections between the various layers within the MLC. C4 pads can be directly soldered onto MLC vias, providing low inductance, direct feed to power and ground planes.
Planarity of the chip attach surface of substrates/carriers is important in order to reliably mount chips via the C4 process to their carriers. One aspect contributing to the non-uniformity of the carrier surface is related to a condition referred to as via-bulge. During firing, the expansion/contraction of the typical conductive paste is not the same as that for the typical dielectric material encompassing the vias and etch lines. Therefore, vias which protrude from the surface and that go into the substrate through many layers will tend to form hills on the carrier's mounting surface, i.e., will produce via-bulge.
Accordingly, what is needed is an approach to routing signal lines that substantially eliminates excessive via depth for high C4 density MLC substrates. The present invention addresses such a need.